It is believed that shock-induced intramural virtual electrodes (IVE) play a critical role in defibrillation. IVE were recently demonstrated in the porcine left ventricle (LV), but their origin remains unknown. Macroscopic optical mapping showed that strong shocks induce IVE of only one polarity, which contradicts theoretical predictions. It is hypothesized that IVE have a microscopic origin and that microscopic positive and negative IVE are spatially averaged during macroscopic optical mapping. This hypothesis was examined by mapping V m responses at the transmural LV surface with increased optical resolution. Rectangular shocks (strength=2-48 V/cm; duration=10 ms) were applied across isolated coronary-perfused porcine LV preparations (n=7) during the action potential plateau and diastole. Shock-induced V m responses were measured at low resolution (LR; 1.2 mm/diode) and high resolution (HR; 0.11 mm/diode). During plateau shocks with strength ≥20 V/cm, LR recordings demonstrated only negative ΔV m extending to the cathodal preparation edge. In contrast, HR recordings from this area as well as from intramural locations revealed both positive and negative ΔV m at all shock strengths. During diastolic shocks, only positive polarizations were observed at LR, but both positive and negative polarizations were detected at HR. In areas of negative polarization, large activation delays were found at HR, whereas LR recordings at these locations demonstrated fast activation. High- and low-resolution optical mapping produced radically different patterns of shock-induced polarization and activation. The occurrence of positive and negative polarizations during plateau and diastolic shocks at high but not low resolution provides evidence for microscopic nature of IVE in LV wall. © 2004 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.